Method and Apparatus for Compensating Nonlinear Distortions in Intensity Modulation-Direct Detection System

1. A nonlinear equalizer in an intensity modulation-direct detection (IM-DD) system, wherein the nonlinear equalizer comprises: a calculation module for calculating, according to nonlinear coefficients and differences between values of an input signal at different times, nonlinear distortions of the input signal; wherein the calculation module calculates the nonlinear distortions of the input signal according to the following formulae: ⁢ Δ t = ∑ i = - N N ⁢ ∑ k = - N i - 1 ⁢ c i , k ⁡ ( x t - i - x t - k ) 2 , or ; Δ t = ∑ i = 1 N ⁢ ∑ k = 0 N ⁢ c i , k ⁢ [ ( x t - i - x t - k ) 2 + ( x t - i - x t + k ) 2 + ( x t + i - x t - k ) 2 + ( x t + i - x t + k ) 2 ] , or ; ⁢ Δ t = ∑ i = 1 N ⁢ ∑ k = 0 i ⁢ c i , k ⁢ [ ( x t - i - x t - k ) 2 + ( x t - i - x t + k ) 2 + ( x t + i - x t - k ) 2 + ( x t + i - x t + k ) 2 ] ; where Δ t is the nonlinear distortions of the input signal, c i,k is the nonlinear coefficient, x t−i , x t−k , x t+i and x t+k are the values of the input signal at different times, and N is a natural number.

2. The nonlinear equalizer according to claim 1 , wherein, when the calculation module calculates the nonlinear distortions of the input signal according to the formulae Δ t = ∑ i = - N N ⁢ ∑ k = - N i - 1 ⁢ c i , k ⁡ ( x t - i - x t - k ) 2 , the calculation module comprises: 2N delayers for delaying the input signal in accordance with a predetermined sampling interval; 2N 2 +N adders for performing a difference operation on the values of the input signal at different times; 2N 2 +N squarers for performing a square operation on the differences between the values of the input signal at different times; and one summator for performing a summation operation on the operation result of each of the squarers.

3. The nonlinear equalizer according to claim 1 , wherein the nonlinear coefficient c i,k is calculated according to the unit impulse response h of a channel: c i , k = 1 4 ⁢ P 0 ⁢ Re ⁡ ( h k ⁢ h i * ) , where, P 0 is DC optical power.

4. A receiver, comprising: a nonlinear equalizer, the nonlinear equalizer comprises: a calculation module for calculating, according to nonlinear coefficients and differences between values of an input signal at different times, nonlinear distortions of the input signal; wherein the calculation module calculates the nonlinear distortions of the input signal according to the following formula: ⁢ Δ t = ∑ i = - N N ⁢ ∑ k = - N i - 1 ⁢ c i , k ⁡ ( x t - i - x t - k ) 2 , or ; Δ t = ∑ i = 1 N ⁢ ∑ k = 0 N ⁢ c i , k ⁢ [ ( x t - i - x t - k ) 2 + ( x t - i - x t + k ) 2 + ( x t + i - x t - k ) 2 + ( x t + i - x t + k ) 2 ] , or ; ⁢ Δ t = ∑ i = 1 N ⁢ ∑ k = 0 i ⁢ c i , k ⁢ [ ( x t - i - x t - k ) 2 + ( x t - i - x t + k ) 2 + ( x t + i - x t - k ) 2 + ( x t + i - x t + k ) 2 ] ; where Δ t is the nonlinear distortions of the input signal, c i,k is the nonlinear coefficient, x t−i , x t−k x t+i , and x t+k are the values of the input signal at different times, and N is a natural number.

5. A transmission system, wherein the transmission system comprises the receiver as claimed in claim 4 .

6. The receiver according to claim 4 , wherein, when the calculation module calculates the nonlinear distortions of the input signal according to the formulae Δ t = ∑ i = - N N ⁢ ∑ k = - N i - 1 ⁢ c i , k ⁡ ( x t - i - x t - k ) 2 , the calculation module comprises: 2N delayers for delaying the input signal in accordance with a predetermined sampling interval; 2N 2 +N adders for performing a difference operation on the values of the input signal at different times; 2N 2 +N squarers for performing a square operation on the differences between the values of the input signal at different times; and one summator for performing a summation operation on the operation result of each of the squarers.

7. The receiver according to claim 4 , wherein the nonlinear coefficient c i,k is calculated according to the unit impulse response h of a channel: c i , k = 1 4 ⁢ P 0 ⁢ Re ⁡ ( h k ⁢ h i * ) , where, P 0 is DC optical power.

8. A transmitter, comprising: a nonlinear equalizer, the nonlinear equalizer comprises: a calculation module for calculating, according to nonlinear coefficients and differences between values of an input signal at different times, nonlinear distortions of the input signal; wherein the calculation module calculates the nonlinear distortions of the input signal according to the following formulae: ⁢ Δ t = ∑ i = - N N ⁢ ∑ k = - N i - 1 ⁢ c i , k ⁡ ( x t - i - x t - k ) 2 , or ; Δ t = ∑ i = 1 N ⁢ ∑ k = 0 N ⁢ c i , k ⁢ [ ( x t - i - x t - k ) 2 + ( x t - i - x t + k ) 2 + ( x t + i - x t - k ) 2 + ( x t + i - x t + k ) 2 ] , or ; ⁢ Δ t = ∑ i = 1 N ⁢ ∑ k = 0 i ⁢ c i , k ⁢ [ ( x t - i - x t - k ) 2 + ( x t - i - x t + k ) 2 + ( x t + i - x t - k ) 2 + ( x t + i - x t + k ) 2 ] ; where Δ t is the nonlinear distortions of the input signal, c i,k is the nonlinear coefficient, x t−i , x t−k , x t+i , and x t+k are the values of the input signal at different times, and N is a natural number.

9. A transmission system, wherein the transmission system comprises the transmitter as claimed in claim 8 .

10. The transmitter according to claim 8 , wherein, when the calculation module calculates the nonlinear distortions of the input signal according to the formulae Δ t = ∑ i = - N N ⁢ ∑ k = - N i - 1 ⁢ c i , k ⁡ ( x t - i - x t - k ) 2 , the calculation module comprises: 2N delayers for delaying the input signal in accordance with a predetermined sampling interval; 2N 2 +N adders for performing a difference operation on the values of the input signal at different times; 2N 2 +N squarers for performing a square operation on the differences between the values of the input signal at different times; and one summator for performing a summation operation on the operation result of each of the squarers.

11. The transmitter according to claim 8 , wherein the nonlinear coefficient c i,k is calculated according to the unit impulse response h of a channel: c i , k = 1 4 ⁢ P 0 ⁢ Re ⁡ ( h k ⁢ h i * ) , where, P 0 is DC optical power.

12. A method for compensating nonlinear distortions in an intensity modulation-direct detection (IM-DD) system, wherein the method comprises: calculating, according to nonlinear coefficients and differences between values of an input signal at different times, nonlinear distortions of the input signal; wherein the step of calculating in particular comprises: calculating the nonlinear distortions of the input signal according to the following formulae: ⁢ Δ t = ∑ i = - N N ⁢ ∑ k = - N i - 1 ⁢ c i , k ⁡ ( x t - i - x t - k ) 2 , or ; Δ t = ∑ i = 1 N ⁢ ∑ k = 0 N ⁢ c i , k ⁢ [ ( x t - i - x t - k ) 2 + ( x t - i - x t + k ) 2 + ( x t + i - x t - k ) 2 + ( x t + i - x t + k ) 2 ] , or ; ⁢ Δ t = ∑ i = 1 N ⁢ ∑ k = 0 i ⁢ c i , k ⁢ [ ( x t - i - x t - k ) 2 + ( x t - i - x t + k ) 2 + ( x t + i - x t - k ) 2 + ( x t + i - x t + k ) 2 ] ; where Δ t is the nonlinear distortions of the input signal, c i,k is the nonlinear coefficient, x t−i , x t−k x t+i and x t+k are the values of the input signal at different times, and N is a natural number.

13. The method according to claim 12 , wherein the nonlinear coefficient c i,k is calculated according to the unit impulse response h of a channel: c i , k = 1 4 ⁢ P 0 ⁢ Re ⁡ ( h k ⁢ h i * ) , where P 0 is DC optical power.